1. Field of the Invention
The present invention relates to a fluid delivery device such as an infusion pump to deliver a fluid with high accuracy.
2. Description of the Related Art
Recently, various infusion pumps have come into use in order to infuse a liquid medicine into human bodies little by little with high accuracy.
Conventional infusion pumps can be classified into four kinds in accordance with the difference in their systems, that is, a syringe pump, a peristaltic (rotor) pump, a finger pump and a bellows pump. Of these, any other than the bellows pump use a motor such as a stepping motor, a rotary solenoid motor, a DC motor, or the like, as a driving source for ejecting a liquid medicine, and adopt a complicated mechanism for controlling the infuse volume of the liquid medicine. Generally, such pumps are therefore heavy in weight and large in size and hence expensive. Therefore, such pumps are usually used on a bedside in a hospital, and are unsuitable for portable or disposable use.
On the other hand, a bellows pump is of a system in which bellows are pushed by use of vaporization pressure of a freon gas to thereby infuse a liquid medicine. However, it is difficult to control the vaporization pressure of the freon gas. Further, this pump has another problem related to the accuracy of the infusion particularly when a very small amount of the liquid medicine is infused over a long period.
One of the present inventors has proposed a device having a pumping function and a gas flow control function in which an electrochemical cell is used for generating an amount of gas which is proportional to the value of a DC current supplied thereto (Japanese Patent No. 1214001). Recently, an electrochemically driven drug dispenser using the principle of the above device has been proposed (H. J. R. Maget, U.S. Pat. No. 4,522,698). This electrochemically driven drug dispenser has an electrochemical cell in which porous gas diffusion electrodes are joined respectively to the opposite surfaces of an ion exchange membrane containing water functioning as an electrolyte. The electrochemically driven drug dispenser uses such a phenomenon that when hydrogen is supplied to an anode of the electrochemical cell and a DC current is made to flow between the anode and the cathode, the hydrogen becomes hydrogen ions at the anode, and when the produced hydrogen ions reach the cathode through the ion exchange membrane, an electrochemical reaction arises to generate hydrogen thereat. That is, the hydrogen generated and pressurized at the cathode is used as a driving source for pushing a piston, a diaphragm, bellows or the like. Alternatively, oxygen may be used in place of hydrogen as a reactor in this electrochemical cell. If the air is used as source of oxygen to be supplied to the cathode, the structure of the drug dispenser may be made considerably simple.
Further, another method has been proposed using electrolysis of water (Unexamined Japanese Patent Publication No. Hei.2-302264), which is an improved type of such an electrochemically driven drug dispenser. In this method, water is contained in an electrochemical cell in which a cathode and an anode are integrally joined to the opposite surfaces of an ion exchange membrane respectively, or a cathode and an anode are joined both onto one of the opposite surfaces of an ion exchange membrane, so that the cathode and the anode are separated and insulated from each other. Thus, hydrogen, oxygen, or a gas mixture of hydrogen and oxygen generated by electrolysis of water when a DC current is made to flow between the electrodes is used as a pressurization source of the drug dispenser.
When a liquid medicine is infused by an electrochemically driven drug dispenser, there is a method in which the liquid medicine with a predetermined volume is put into a vessel provided with a flexible diaphragm, and the diaphragm is then pushed by the pressure of a gas so as to infuse the liquid medicine. As a specific example of this method, a structure has been proposed in which a flexible diaphragm is disposed in a part of an annular vessel, a liquid medicine is put into a vessel portion surrounded by a wall portion of the vessel and the flexible diaphragm, and a gas generated from an electrochemical cell is supplied to a gap formed between another wall portion of the vessel and the flexible diaphragm to thereby infuse the liquid medicine (H. J. R. Maget, U.S. Pat. No. 4,902,278). In such a structure, when different kinds of liquid medicines are used in one device, it is necessary to cleanse the inside of the vessel every time, and therefore only one kind of a liquid medicine can be used in one device in practical use.
As a practical matter, however, it is impossible to manufacture such a drug dispenser easily and inexpensively because of its complicated structure. In addition, particularly in the case where a liquid medicine is to be delivered, a disinfection or sterilization of the whose system is inevitable. This operation is very difficult to perform in the above-mentioned structure.
There has been further considered to a device in which a liquid medicine or blood is charged in advance into a fluid reservoir consisting of a flexible bladder having a fluid delivery pipe. This fluid reservoir is put into a closed vessel, and gas generated from an electrochemical cell is introduced into the closed vessel. The pressure in a space defined by the inner wall of the closed vessel and the outer surface of the bladder is increased to push the bladder to thereby deliver a fluid. In this device, when a disinfected fluid such as a liquid medicine, blood or the like is to be delivered into a human body, such a fluid must not come into contact with the air or the like at all. However, the consideration of sealing the vessel into which the fluid reservoir is received, or sealing the closed vessel in the condition that a fluid delivery pipe is led to the outside of the closed vessel is troublesome. For instance, an operation of screwing through an O-ring, or the like, is troublesome.